This invention relates generally to radiological well logging methods and apparatus for investigating the subsurface earth formations traversed by a borehole and, more particularly, to an improved method and apparatus for pulsed neutron gamma ray logging wherein gamma rays resulting from neutron inelastic scattering and thermal neutron capture are selectively detected and the spectral distribution of the gamma rays is determined and utilized for direct porosity identification.
A number of well logging techniques in use come under the general classification of neutron induced radiological well logging. In their basic form, these techniques involve irradiating subsurface earth formations with high energy neutrons and then determining the effect of formation constituents on the irradiating neutrons by measuring gamma rays produced by inelastically scattered neutrons or the gamma rays resulting from thermal neutron capture. In the case of inelastic scattering, part of the energy lost by the neutron is released in the form of gamma rays at the instant of collision. A capture reaction is one of which the thermalized neutron is absorbed into the nucleous of an atom and ceases to exist independently. In most such capture reactions, one or more high energy gamma rays are generated.
The detection of these radioactive signals returning to the borehole can provide information as to the porosity, lithology and presence or absence of hydrocarbons within the formation. One property of subsurface formations of particular interest is porosity, which in rocks is space not occupied by solid material. In subsurface formations this pore space is ordinarily occupied by fluids which are hydrogenous in composition.
Several prior art methods have been proposed for porosity evaluation of subsurface formations by means of bombarding the formations with neutrons. An example of such prior art instrumentation is described in U.S. Pat. No. 3,621,255, issued to R. J. Schwartz, where the neutron population returning to the borehole is sampled by a pair of neutron detectors spaced at different distances on the longitudinal axis from the neutron source. Due to the necessity of providing high counting rates to reduce statistical deviation, the source-detectors spacings must be shorter than would be desired. As a result of the short spacings the counting rates of the two detectors are affected unequally by changes in porosity. Thus, this instrument will only provide a mesurement related to porosity and not a measurement of true porosity.
Another method of porosity determination using a pulsed neutron source is illustrated in U.S. Pat. No. 4,239,956, issued to D. W. Oliver and R. B. Culver, and assigned to the assignee of the present invention. The patent discloses a system whereby a carbon/oxygen ratio and a hydrogen/iron ratio are derived from fixed energy ranges within the capture gamma ray spectrum. These ratios are then combined to obtain a direct porosity indicator. Due to the complexities involved in obtaining the various abovedescribed measurements the disclosed method has been determined not to be the simplest and most reliable means of identifying formation porosity.
Accordingly, it has been proven difficult to establish an easily obtainable yet reliable measurement of true formation porosity. The present invention overcomes the deficiencies of the prior art by providing an improved method and apparatus for direct porosity identification derived from the measurement of gamma rays resulting from neutron inelastic scattering and thermal neutron capture.